BTRFS: Read only filesystem while mounted read write

While doing my monthly backups I got a little bit confused. I found an old backup of an LXC container which was not needed anymore. But I was not able to delete it:

rm: cannot remove ‘container_backups/XXX/rootfs/var/log/private’: Read-only file system

Checking with mount the backup filesystem was mounted rw and dmesg also did not indicate a btrfs problem. Missing privileges were also not an issue as I was logged in as root. But then I remembered that I was experimenting with btrfs subvolumes and used the send/receive function for backups. And there you need in some cases set a ro label on the subvolume. My guess was that this label was still be present.

btrfs property get container_backups/XXX/rootfs/

Indeed it was still set. Simply setting it to ro = false is apparently not a good solution:

btrfs property set container_backups/XXX/rootfs/ ro false
ERROR: cannot flip ro->rw with received_uuid set, use force if you really want that

Did I care? No. The data should be deleted anyways. I inserted the -f switch and dit it anyway.
Conclusion: With current filesystems a rw mount flag does not always mean that it’s always writeable!

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